To Top Up the Diesel Tank or Not?

[Sandy]

I think that most of us on here, are talking about filling the tank for the winter, and it not being used for maybe six months. In that case heat from the return/excess fuel line, is entirely irrelevant. If the tank is pressed full, there will be little or no air present to cause oxidation. Even if not pressed full, but say an inch from the top, there will be much less volume of air. I know that you will say that the tank is vented, but if there is little or no air in the tank, what will encourage venting?
Anyway, the chances are that by the Spring, fuel will be more expensive.

How much fuel do you use in a year?

I have a 100l tank and my average usage is 25l per annum or in other words I have the capacity for five years storage.

I found the following YouTube video informative.

But then I had the fuel bug last summer! Since then I've had the tank out, steam cleaned, fuel lines replaced (they were the original ones and at 35 years old about time they were replaced) and moved the primary filter to somewhere I could access it easily (its also a top loading filter so that the job can be done without needing to be a contortionist).

I've also moved to white as the turn over of Red at the pump is tiny and I can remove the fuel from the boat and use it in the car.

 

[JumbleDuck]

Is he trying to say that nobody in the UK ever gets condensation problems?

People certainly get water problems; whether they get condensation problems is quite another matter.

Extreme case: suppose that a 100 litre tank is warmed ever day to 10^oC and then cooled every night to 0^oC. Total air ingested each cycle will be 10/283 x 100 litres = 3.5 litres. On average it's going in at 5^oC, and if it's fully saturated will have at most 7g/m³ of water in it, so that's 7g x 3.5/1000 = 0.025g per cycle. Repeat this 150 times over the winter and your 100 litre tank has acquired a horrifying 3.7cc of water or 37ppm. As supplied diesel to standard EN-590 can come with 200ppm in it.

So in exceptional and artificial conditions, condensation might increase the amount of water in diesel fuel by one fifth.

If significant amounts of water are getting in, there must be some other mechanism. My bet's on leaky fillers.

 

[superheat6k]

Most boat tanks are situated inside a boat's structure with an air gap all around, with the only permanent opening being a small bore vent pipe. Even on very cold days the insulating effect of the boat itself drastically reduces the temperature change affecting the tank walls.

As the fuel is drawn air will slowly enter via the vent and if the tank then cools the moisture held naturally in the air will condensate out, but this will only happen once and even for a large tank the volume would be minute. This is by no means the same as a cold window or car on a cold morning where the air is continually moving over the cold surface, and hence giving up relatively large volumes of water. Once the air into the tank has stabilised to a lower humidity that's it, there will be very little fresh air exchange through the breather pipe, but perhaps a simple Winter precaution could be a soft bung of cotton wool stuffed gently into the vent opening.

However, the major source of water entry into a tank during Winter is often entirely ignored - the flush deck filler cap. These often have a very tired O ring seal, and sit for months being deluged with rain and if out on the boat, seawater. A small blob of waterproof grease easily seals the threads and prevents the cap from corroding into place. OK it makes your fingers a little mucky when next refuelling, a tiny price to pay for a dry tank, contaminant free tank.

 

[superheat6k]

People certainly get water problems; whether they get condensation problems is quite another matter.

Extreme case: suppose that a 100 litre tank is warmed ever day to 10^oC and then cooled every night to 0^oC. Total air ingested each cycle will be 10/283 x 100 litres = 3.5 litres. On average it's going in at 5^oC, and if it's fully saturated will have at most 7g/m³ of water in it, so that's 7g x 3.5/1000 = 0.025g per cycle. Repeat this 150 times over the winter and your 100 litre tank has acquired a horrifying 3.7cc of water or 37ppm. As supplied diesel to standard EN-590 can come with 200ppm in it.

So in exceptional and artificial conditions, condensation might increase the amount of water in diesel fuel by one fifth.

If significant amounts of water are getting in, there must be some other mechanism. My bet's on leaky fillers.

Out of interest can I ask where the equation with the figure 283 is derived from ?

 

[lw395]

Out of interest can I ask where the equation with the figure 283 is derived from ?

0degC is 273 Kelvin. 10 degC is 283K Volume of an ideal gas is proportional to absolute temperature so the ratio of hot and cold masses of air is 273 to 283, assuming the pressure stays constant.

 

[superheat6k]

0degC is 273 Kelvin. 10 degC is 283K Volume of an ideal gas is proportional to absolute temperature so the ratio of hot and cold masses of air is 273 to 283, assuming the pressure stays constant.

I thought that was where the figure came from, but the reduction in volume in the equation suggests that at -273oC or 0 Kelvin, the Gas volume becomes zero. Is that correct ? (I know that in reality nothing can reach absolute zero). I guess I need to swat up on Boyle's law.

 

[JumbleDuck]

I thought that was where the figure came from, but the reduction in volume in the equation suggests that at -273oC or 0 Kelvin, the Gas volume becomes zero.

It would if it didn't condense and then, except in the case of helium, freeze first. The universal gas law is OK, but doesn't take account of phase changes or of atoms/molecules getting very close together.

 

[NormanS]

How much fuel do you use in a year?

I have a 100l tank and my average usage is 25l per annum or in other words I have the capacity for five years storage.

I found the following YouTube video informative.

But then I had the fuel bug last summer! Since then I've had the tank out, steam cleaned, fuel lines replaced (they were the original ones and at 35 years old about time they were replaced) and moved the primary filter to somewhere I could access it easily (its also a top loading filter so that the job can be done without needing to be a contortionist).

I've also moved to white as the turn over of Red at the pump is tiny and I can remove the fuel from the boat and use it in the car.

I'm not sure why you want to know how much fuel I use, but I carry 455 litres, divided into twin tanks. Annual consumption is about 350 litres. Touch wood, I've never had the slightest sign of diesel bug, but as my tanks are proper fuel tanks with drainable water / dirt sumps, they never have a fuel / water interface, which can encourage growth of the "bug". I also burn about 100 litres of kero in an Eber.

P.S. I think you need to check your arithmetic, or you might get a nasty surprise in year 5.

 

[maby]

How much fuel do you use in a year?

I have a 100l tank and my average usage is 25l per annum or in other words I have the capacity for five years storage.

....

Our tank is about 150 litres if memory serves me correctly and I will have to refill it at least once each winter - but most of that goes into the Webastospacher.

 

[lw395]

I thought that was where the figure came from, but the reduction in volume in the equation suggests that at -273oC or 0 Kelvin, the Gas volume becomes zero. Is that correct ? (I know that in reality nothing can reach absolute zero). I guess I need to swat up on Boyle's law.

Gases generally cease to behave as 'ideal gases', at some temperature. It's an idealised model which would work for gases with infinitely small molecules.
It fails when the molecules start to get too close together due to high pressure or low temperature, or the gas starts to condense.
But for real gases at sensible pressures and temperatures, if you plot the volume against temperature at a constant pressure and extrapolate to zero volume, that line is crossed at -273.15degC. Near enough for dockyard work anyway!